The present invention relates to a control system for automatically changing between two-wheel drive and four-wheel drive modes of operation in an off-road vehicle, such as an agricultural tractor.
In an agricultural tractor provided with a conventional clutch to engage four-wheel drive, when the clutch is engaged, the front wheels are normally driven by about 2% faster than the rear wheels. This speed differential is caused by the geometry of the transmission members fitted to the front wheels. In the field, that is when the tractor is travelling over agricultural ground, because of the low speed at which the tractor normally runs and because of the fact that the agricultural ground offers a low adhesion to the tires, it is established that with the four-wheel drive transmission engaged all the wheels may slip even by up to 10-15% with respect to the speed imposed by the engine of the tractor so that the difference in speed between the front wheels and the rear wheels is not noticed.
On metal ed road surfaces, however, because the tractor tends to be driven at higher speeds and because there is better adhesion between the tires and the road surface, there is a considerable slipping of the front wheels on the ground when four-wheel drive is engaged. The front wheels slip in preference to the rear wheels because in tractors most of the weight is supported by the rear wheels and the frictional force on the ground at the rear wheels is greater than at the front wheels.
Because the slipping of the front wheels on metalled road surfaces causes a high degree of wear on the front tires, it is preferred to disengage the clutch and revert to two-wheel drive when driving on good road surfaces.
Furthermore, on bends, especially on those of a short radius, two-wheel drive is preferred to four-wheel drive because even when the front axle differential is not locked, the front wheel on the outside of the bend tends to travel up to 20% faster than the rear wheels so that, if four-wheel drive would be engaged, this front wheel would slip, again causing considerable wear on the tire.
For these reasons, it is desirable in a tractor to be able to select between two-wheel and four--wheel drive in dependence upon the road surface on which the tractor is driven. Ideally, the tractor should have four-wheel drive engaged only under poor road adhesion conditions when the rear wheels slip with respect to the ground.
In the prior art, it is known to provide a transmission in which two-wheel and four-wheel drive can be selected by the operator of the tractor, but this requires the operator to have the skill to know when to change between four-wheel and two--wheel drive for the best traction and minimum tire wear.
To simplify the task of the operator, it has already been proposed to provide a transmission with a control system that can automatically select four-wheel drive, whenever required to reduce slipping of the rear wheels.
The Applicants' European Patent EP-A-0.432.549 which is believed to represent the closest prior art to the present invention, discloses a control system for automatically engaging four-wheel drive in a vehicle transmission. The transmission comprises a first drive shaft for transmitting drive from the engine to two rear wheels and a second drive shaft for transmitting drive to two front wheels. A clutch acts, when engaged, to couple the two drive shafts for rotation with one another and the drive transmission geometry is such that, with the clutch engaged, the transmission causes the said front wheels to be driven slightly slower than the rear wheels. In the second drive shaft, a special coupling is provided which comprises toothed drive and driven members that mesh with one another with lost motion, that is to say, the two members can rotate one relative to the other through a limited angle. Relative rotation of the drive and driven members of the coupling is sensed by an element adapted to move axially between two end positions as the lost motion in the coupling is taken up to indicate the direction in which torque is being transmitted through the coupling. A sensor detects the end positions of the latter element and generates an electrical signal that is used by an electronic processing unit to control engagement or disengagement of the clutch.
The principle on which the above proposal is based is that, when the rear wheels slip in two-wheel drive, the drive member of the lost motion coupling will try to rotate faster than the driven member, the speed of the latter corresponding to the road speed of the vehicle. When on the other hand four-wheel drive is engaged and the wheels are not slipping on the ground, then the front wheels will try to roll on the ground faster than the speed at which they are being driven by the transmission with the result that the driven member will try to rotate faster than the drive member and will apply a reverse torque to the engine through the lost motion coupling. During the reversal of the direction of the torque through the coupling, the backlash or lost motion in the coupling is taken up resulting in an angular relative displacement between the drive and the driven members which is translated into an axial displacement of the sensor element. As already mentioned, in response to the movement of the sensor element in the one or other direction, the electronic processing unit controls the four-wheel drive clutch to either engage or to disengage.
Although the above arrangement has proven to be extremely efficient in practice, it nevertheless suffers from the disadvantage that, apart from the four-wheel drive clutch, a separate lost motion coupling with additional sensor controlling an electronic processing unit is needed to automatically switch between two-wheel drive and four-wheel drive. It will be appreciated that this is not cost-effective and complicates the assembly of the parts and the calibration of the arrangement.